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Specific Requirement of Potassium for Light-Activated Opening of Stomata in Epidermal Strips ¹

^a Laboratory of Plant-Water Relations, Department of Water Science and Engineering, University of California, Davis, California 95616

The effect of various ions on stomatal opening was studied in isolated epidermal strips of Vicia faba L. Stomata in strips floating on 10 mM KCl and in CO₂-free air opened in light, closed in subsequent darkness, then opened fully again when illuminated. A light-activated highly specific effect of K^- (and Rb⁺) on opening was found. When strips were floated on high concentrations (50 or 100 meq/liter) of Li⁺, Na⁺ or Cs⁺, stomata opened but light had very little effect on the concentrations required for opening. With K⁺, the opening produced in the dark was the same as with the other alkali ions. Light, however, lowered more than 100-fold the concentration of K⁺ required for maximal opening. Thus only the effect of K⁺ (and Rb⁺) was greatly accentuated by light. NH₄⁺ and Mg²⁺ did not produce opening.

No specific anion is required in association with K⁺. Opening was the same when Cl^-, Br, and NO₃^- were used as counter ions, but was less when SO₄^2- was used, particularly at higher concentrations and in the dark.

The results are discussed in relation to the recent proposal that the basis for stomatal opening is K⁺ uptake in amounts sufficient to act as an osmotic agent. This work also demonstrates, for the first time, a physiological process specifically requiring K⁺. Assuming that ion uptake is an integral part of stomatal opening, guard cells would appear to have an ion uptake mechanism of a degree of specificity previously unknown in higher plants.

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